1. Field of the Invention
The present invention relates to a magnetron, and more particularly, to a structure for mounting a noise removing filter circuit which prevents noise waves produced by the magnetron body from leaking out as unneeded radiation through a cathode circuit portion.
2. Description of the Prior Art
FIG. 1 is a longitudinal section showing an example of the conventional magnetron used in an electronic range. Reference numeral 1 denotes the thermionic valve body of the magnetron with cooling fins 2 being attached to the periphery of the valve by press fitting. Reference numeral 3 denotes a ferrite permanent magnet which produces a static magnetic field along the valve axis in the space of action accommodated within the thermionic valve body of the magnetron, and a yoke 4 supports this permanent magnet 3 so as to enclose the same on all sides, thereby forming a magnetic circuit. Holes are drilled in the upper and lower portions of the yoke 4, and an antenna portion for drawing the output of the above-mentioned thermionic valve body 1 and a cathode input portion for feeding electric power to the cathode are disposed so as to pass through the lower and upper holes, respectively. A filter case 5 is disposed so as to surround the cathode input portion, housing a through-type capacitor 6 constituting part of this filter which is supported by the case wall through which it passes. Also, a choke coil 7 constituting part of this filter is connected at one end to the capacitor 6 and at the other end to a cathode terminal 8. The upper opening of the case 5 is sealed by a lid 9. In this construction, the choke coil 7 and the capacitor 6 mainly perform the function of preventing the leakage of radiation contained by the television bandwidth which is propagated through an input line while the magnetron is in operation, and the filter case 5, sealed by the lid 9, possesses the function of trapping other unneeded waves such as the higher harmonics of the fundamental wave (normally 2.45 GHz) produced by the magnetron.
FIG. 2(a) is a fragmentary view as viewed in the direction shown by the arrow Z in FIG. 1, illustrating the manner wherein the capacitor 6 of the filter is mounted in the filter case 5, in which symbol 6a denotes a mounting plate for the capacitor 6, and symbol 6b an insulating tubular portion which covers a through conductor 6c. In order to secure the mounting plte 6a to the case 5, burring portions 5a are provided in the positions corresponding to the respective corners of the mounting plate 6a. FIG. 2(b) is an enlarged fragmentary cross-sectional view of the burring portion 5a cutaway longitudinally along the axis thereof, wherein the mounting plate 6a having holes which receive the respective burring portions 5a of the filter case 5 as illustrated, is fixed to the wall of the filter case 5 by caulking the end 5b of the respective burring portion 5a.
This conventional method of mounting the filter causes the following problems.
(a) Since the mounting plate 6a for the capacitor is fixed only at the four burring portions 5a, some interstices are formed between the filter case wall and the mounting plate 6a over the length between the burring portions, and thereby, shielding effects are deteriorated particularly above the fifth harmonic which has a high frequency.
(b) The filter case 5 and the capacitor mounting plate 6a tend to suffer from curvature, and the interstices remain considerably uneven after caulking.
(c) Unneeded radiation leaks out of the holes of the burring portions 5a for joining the mounting plate 6a to the filter case 5.
Hence, the filter case 5 and the capacitor mounting plate 6a do not conform with each other perfectly, thus making it impossible for them to be joined closely to each other owing to the fact that either may suffer slight curvature. In addition to these problems, since the commencement of satellite broadcasting, the leakage of higher harmonics which are components of the unneeded waves emanating from the magnetron has recently become a target for strict regulation. In spite of these critical situations, however, it has been found that prior art magnetrons entail the risk of leakage not only out of the above-described slight interstices between joint faces but also through the holes of the burring portions 5a of the filter case 5 which were described with reference to FIG. 2.